Transcranial Magnetic Stimulation (TMS) and Repetitive Transcranial Magnetic Stimulation (rTMS, a variant of TMS in which electromagnetic fields are produced in trains of multiple short pulses) can trigger neuronal firing in selected brain regions. TMS-induced neuronal firing may be therapeutically effective for at least one psychiatric condition, major depression. Because a magnetic field always diminishes as a function its distance from the source, however, TMS and rTMS instrumentation is currently limited by the inability to focus the magnetic fields at depths without discomfort to the patient.
Attempts have been made to focus electromagnetic energy into deep structures of the brain without overwhelming superficial structures. For example, efforts have been made to simultaneously use multiple coils such that the magnetic fields converge at a chosen point (see Sackheim, H A. Magnetic Stimulation Therapy and ECT (Commentary) Convulsive Therapy, 1994, 10(4): 255-8). Even if feasible for achieving greater penetration, however, the difficulty of coordinating the fields from multiple coils (e.g., adjusting for a specific target) makes this approach sub-optimal.
U.S. Pat. No. 6,572,528, describes the use of an adaptation of a 1.5 Tesla MRI scanner to produce some form of transcranial magnetic stimulation. In this machine, the largest magnet (the solenoid) remains stationary and at steady state, while the programmable magnets (e.g., the head coil and the gradient coil) are of relatively low field strength. Consequently, such a configuration may not be able to selectively stimulate targeted deep brain structures while sparing superficial structures.
A variety of new electromagnet configurations have been developed by the Helsinki group (Ruohonen, J, Dissertation for Doctorate of Technology, Helsinki University of Technology, Espoo, Finland, 1998), which may be useful in the context of TMS for reaching to deeper structures. However, these magnets pass the majority of their energy through interposed proximal tissue and thus can have undesired side off-target effects when used for deep TMS.
Specially-shaped coils that provide more gradual decrease in magnetic field as a function of distance from the coil have been reported (see, e.g., Roth, Y; Zangen, A; Hallet, M; Journal of Clinical Neurophysiology, 2002, 19(4): 361-370). In addition, multiple-coil configurations have been tested for the ability to stimulate deep brain structures (see George, M S Stimulating the Brain, Scientific American, editor's inset window, page 72 September 2003). Such devices, however, are likely to be inflexible and may require different coil arrays to target different brain regions or even to target the same brain region in two different individuals.
Another mechanism of targeting magnetic fields to specific structures is to attach mechanical or computerized stereotactic neurosurgical image guidance systems to TMS coils (e.g., STEALTH STATION by Surgical Navigation Technologies, Inc., Broomfield Colo.). These methods also suffer from the problem of providing strong and potentially harmful magnetic fields to intervening tissue in an effort to provide ample stimulation to the targeted structures.
Consequently, there remains a need for devices and methods by which magnetic fields can be directed so as to selectively affect deep-targeted structures while leaving superficial structures relatively undisturbed.